The present invention relates to optical communication systems, and more particularly to amplifiers, specifically Erbium Doped Fiber Amplifiers (EDFA), used in Wavelength Division Multiplexing (WDM) optical communication systems.
EDFAs are used in WDM optical communication systems for amplifying many wavelength channels simultaneously in the Erbium doped fiber Gain band. There are two types of EDFAs for WDM: Fixed Gain (FG). optical amplifiers, which can be operated gain flattened and with low Noise Figure only at a pre-determined gain, as described for example in U.S. Pat. No. 5,225,922 to A. R. Charplyvy et. al; and Variable Gain (VG) amplifiers, which can be operated with low noise figure and gain flattened over wide range of gains, as described for example in U.S. Pat. No. 5,812,710 to Y. Seguya et al, U.S. Pat. No. 6,049,413 to M. G. Taylor et al., and U.S. Pat. No. 6,611,641 to U. Ghera et al.
Variable gain amplifiers are usually preferred in long distance systems that normally comprise a plurality of amplifiers, because usage of such an amplifier allows the user to operate the system with an overall low noise figure (NF). The drawback of these amplifiers is that they are much more complicated in their optical design, as discussed in the U.S. patents to Seguya, Taylor and Ghera above. Moreover, VG amplifiers are much more expensive than FG amplifiers. On the other hand, FG amplifiers, while inexpensive, suffer from a number of limitations, chiefly the fixed gain which causes non-optimal performance in a WDM systems based on a multiplicity of wavelengths.
There is thus a widely recognized need for, and it would be highly advantageous to have, WDM amplifiers and amplification systems that are inexpensive and simple like FG amplifiers, yet capable to endow the WDM system with the advantages of a VG amplifier, e.g. gain flattening over a wide range of gains.